1. Field of the Invention
The present invention relates generally to pressure and temperature sensing gauge instruments, and, more particularly, to a dual Bourdon tube gauge which measures temperature and pressure in a single gauge housing.
2. Description of the Prior Art
The use of Bourdon tubes to measure variations in pressure is well known in the prior art. Typically, a Bourdon tube is constructed of a flattened tube of spring steel or bronze, bent in a coil form or "C" shape. One end of the Bourdon tube is stationary and has an open inlet, which communicates with the source of pressure, while the other end is sealed and allowed to freely move in proportion to the difference in pressure between the interior and exterior of the tube. As the pressure within the tube increases, the tube tends to straighten, resulting in a greater deflection. The amount of deflection of the free end of the tube is translated, via mechanical or electrical means, into a calibrated output indication of pressure.
It is also known in the prior art to adapt Bourdon tubes for use in temperature measuring instruments. Such temperature gauges generally consist of a closed system employing a hollow cylindrical bulb connected to the Bourdon via a capillary tube. The system is filled with a liquid or gas. The Bourdon tube responds to changes in fill volume (liquid filled system) or changes in pressure (partial liquid filled or gas filled system) related to the changes in temperature at the bulb.
In its simplest form, a typical prior art pressure gauge employs a multiturn spiral Bourdon coil having a pointer mounted directly to its free end. For example, U.S. Pat. No. 3,641,820, to Bissell, discloses an analog pressure gauge employing a Bourdon tube having, at its free end, an integral pointer which moves relative to a calibrated dial plate in response to changes in pressure to which the gauge is exposed. Further typifying analog pressure gauges is U.S. Pat. No. 4,361,046, to Wetterhom, which discloses the use of a mechanical movement or linkage adapted to amplify and transmit the displacement motion of the Bourdon tube to an output indicating pointer. These prior art gauges, however, were capable of measuring only a single parameter.
U.S. Pat. No. 3,905,237(the '237patent), to Smalarz et al., discloses a temperature-compensated pressure gauge. The '237patent employs a first Bourdon tube coupled to a first indicating pointer to display pressure in the conventional fashion and a second Bourdon tube mechanically coupled to a second dial indicator to continuously define an acceptable operating zone over a predetermined temperature range. This gauge, however, is not capable of concurrently displaying both pressure and temperature information.
It is desirable, particularly in boiler gauge applications on hydronic heating systems where it is typically necessary to monitor both water temperature and system pressure, to use a gauge capable of indicating pressure and temperature on a single gauge face. Some prior art gauges have demonstrated the ability to measure both temperature and pressure in a single unit. These gauges, however, generally employ bimetallic temperature sensing elements and C-shaped Bourdon tubes with delicate mechanical gear or linkage arrangements to couple Bourdon tube displacement to an indicating means.
A significant disadvantage of this type of mechanical translation arrangement is that the complexity of fabrication and assembly of the gauge is greatly increased, thereby increasing cost. Additionally, because of the greater number of moving components, reliability is considerably sacrificed. Another disadvantage of using a linkage arrangement is that overall gauge accuracy is diminished as a result of friction introduced by the moving linkage components. Moreover, since the linkage acts to amplify Bourdon tube displacement, there is enhanced sensitivity to vibration and jarring, thereby creating an additional source of error.
Consequently, there is a need to provide a gauge which is capable of simultaneously displaying pressure and temperature information in a single gauge housing that is reliable and is inexpensive to manufacture and repair.